Problem: Multiply the following complex numbers, marked as blue dots on the graph: $[\cos(\frac{3}{2}\pi) + i \sin(\frac{3}{2}\pi)] \cdot [4(\cos(\frac{5}{4}\pi) + i \sin(\frac{5}{4}\pi))]$ (Your current answer will be plotted in orange.)
Solution: Multiplying complex numbers in polar forms can be done by multiplying the lengths and adding the angles. The first number ( $\cos(\frac{3}{2}\pi) + i \sin(\frac{3}{2}\pi)$ ) has angle $\frac{3}{2}\pi$ and radius $1$ The second number ( $4(\cos(\frac{5}{4}\pi) + i \sin(\frac{5}{4}\pi))$ ) has angle $\frac{5}{4}\pi$ and radius $4$ The radius of the result will be $1 \cdot 4$ , which is $4$ The sum of the angles is $\frac{3}{2}\pi + \frac{5}{4}\pi = \frac{11}{4}\pi$ The angle $\frac{11}{4}\pi$ is more than $2 \pi$ . A complex number goes a full circle if its angle is increased by $2 \pi$ , so it goes back to itself. Because of that, angles of complex numbers are convenient to keep between $0$ and $2 \pi$ $\frac{11}{4}\pi - 2 \pi = \frac{3}{4}\pi$ The radius of the result is $4$ and the angle of the result is $\frac{3}{4}\pi$.